Transfer RNA is produced as a precursor molecule that needs to be processed both at its 3'and 5'ends. Ribonuclease P, or RNase P, is the only endonuclease responsible for processing the 5'end of tRNA by cleaving a precursor and leading to tRNA maturation. It is composed of a large RNA molecule and at least one protein and it has been identified in all organisms. It was one of the first catalytic RNA molecules discovered and its study has been pivotal to our understanding of the role of RNA molecules in catalysis. RNase P is a true multi-turnover ribozyme that recognizes its substrate in trans and one of only two universal ribozymes. The knowledge of the structure and function of RNase P promises to provide important and relevant information on a key ribozyme involved in a central cellular process common to all organisms and also to further our understanding of the structure and function of large RNA molecules. This proposal is concerned with the structure and function of RNase P. In the past few years we have made substantial progress, including solving the structure of the intact RNA component of Thermotoga maritima RNase P. This structure provided important structural information on the RNA component of RNase P and allowed us to make crucial observations regarding the function of this universal ribozyme and relate the structure to the wealth of existing biochemical data. For the next funding period we propose to continue and expand our studies of RNase P. The specific aims for this proposal are: 1) to determine the three dimensional structure of a complex of the RNase P holoenzyme with tRNA, 2) to extend our structural studies of the RNA component of T. maritima RNase P to higher resolution and, 3) to study the role of the universally conserved regions in the structure of RNase P and the implications for RNA recognition and cleavage. The work is based on a combination of molecular biology and biochemical methods to produce and characterize the molecules that we require for our work, and X-ray crystallography to solve their atomic structures.